The present invention relates to an accelerator for accelerating charged-particle beam and producing the beam to be used, a method of producing the beam, and a medical system using the beam.
A conventional accelerator system and method of producing the charged particle beam from the accelerator system are described in JP No. 2,596,292.
As in the publication Ser. No. 2,596,292, the charged particle beam from a preaccelerator is made incident to the following-stage accelerator. The following-stage accelerator accelerates the charged particle beam up to the energy to be necessary for treatment, and produces the beam. The charged particles circulate while vibrating left and right or up and down. There are called betatron oscillations. The number of vibrations per orbit of the betatron oscillation is called tune. Two four-pole electromagnets for convergence and for divergence are used, making the tune close to an integer+⅓ or an integer+⅔ or an integer+xc2xd. At the same time, a multiple-pole electromagnet for causing resonance provided on the circular orbit is excited, thereby suddenly increasing the amplitude of the betatron oscillations of the charged particles having more than a certain betatron oscillation amplitude, of a large number of the charged particles that go round. This sudden amplitude increase phenomenon is called resonance of betatron oscillation. The threshold of the amplitude of the betatron oscillations at which the resonance occurs is called stability limit, the value of which changes depending on the relation between the intensities of the resonance generating multi-pole magnetic field and the four-pole magnetic field. The resonance caused when the tune made close to an integer+xc2xd is called second order resonance, and the resonance when the tune made close to an integer+⅓ or+⅔ is called third order resonance. A description will hereinafter be made of a case in which the tune is made close to an integer+⅓ at the third order resonance. The value of the stability limit of resonance decreases as the deviation of tune from an integer+⅓ diminishes. Thus, in the prior art, while the intensity of the resonance generating multi-pole electromagnet is kept constant, the tune is first approached to an integer+⅓, and made constant, namely, the field intensity of the four-pole magnet is maintained constant as well as the intensities of the deflecting electromagnet and resonance generating multi-pole electromagnet are kept constant. Then, a high-frequency electromagnetic field having a plurality of different frequency components or a frequency band is applied to the beam, increasing the betatron oscillation amplitude to generate resonance. The beam is produced from the extracting deflector by making use of the increase of betatron oscillation due to the resonance. The extracted ion beam is transported by use of an electromagnet of an ion beam transport system to a treatment room.
An extracting-purpose high-frequency source used in the conventional accelerator is described in JP-A-7-14,699. The charged particle beam has its tune changed depending on the betatron oscillation amplitude under the action of the resonance generating multi-pole electromagnet. Therefore, the high frequency for beam extraction is required to have a frequency band, or a plurality of different frequency components. In the prior art, such high frequencies, are applied to the charged particle beam, as to have a frequency band of about several tens of kHz including the product of the tune""s decimal fraction and revolution frequency of the charged particle beam extracted from the cyclic type accelerator.
The charged particle beam emitted from the accelerator, as described in JP-A-10-118,204, is transported to a treatment room where an irradiator for treatment is provided. The irradiator has a scatterer for increasing the beam diameter, and a beam scanning magnet for making the diameter-increased beam circularly scan. The circular scanning of the beam increased in its diameter by this scatterer acts to flatten the integrated beam intensity inside the locus of the scanning beam center. The beam with the intensity distribution flattened is made coincident in its shape with the diseased part by a patient collimator before being irradiated on the patient.
In addition, though different from the above, a small-diameter beam may be used and scanned for its shape to comply with the diseased part by use of the beam scanning electromagnet. In this small-diameter beam scanning method, the current to the beam scanning electromagnet is controlled to irradiate the beam at a predetermined position. The high frequencies are stopped from being applied to the beam after confirming the application of a certain amount of irradiation by a beam intensity monitor, thus the beam being stopped from emission. After the stopping of beam irradiation, the current to the beam scanning electromagnet is changed to change the irradiation position, and the beam is again irradiated in a repeating manner.
Thus, in the conventional medical accelerator system, before being irradiated, the beam is increased in its diameter by the scatterer and circularly deflected to scan so that the integrated intensity distribution in the region inside the scan circle can be flattened. In this beam scanning irradiation, to flatten the intensity distribution, it is desired to reduce the change of the beam intensity, and particularly to decrease the frequency components ranging from about tens of Hz to tens of kHz. However, in the conventional medical accelerator system, since the high frequencies to be applied to the charged particle beam have a frequency band, or a plurality of different frequencies for the emission, the beam emitted from the accelerator has frequency components ranging from about tens of Hz to tens of kHz, and the intensity thereof is changed with lapse of time. Therefore, in order to obtain a uniform irradiation intensity distribution, it is necessary to properly select the circular scanning speed according to the change of beam intensity with time, or to flatten the irradiation intensity distribution by selecting a scanning frequency deviated from the frequency of the beam intensity change. The beam intensity change problem can be solved by much increasing the circular scanning frequency, but the cost of the scanning electromagnets and power supply is greatly increased. Moreover, when the beam intensity change with time is great, the conditions such as reproducibility and stability of the current to the scanning electromagnet, which are necessary to suppress the change of the irradiation field intensity distribution to within an allowable range, are severer than in the case where the beam intensity change with time is small.
Moreover, in the prior art, even though the canning beam diameter is large or small, the beam intensity change with time makes it necessary to increase the time resolution of the beam intensity monitor to confirm a predetermined irradiation intensity distribution.
Accordingly, it is an object of the invention to provide an accelerator capable of suppressing the change of the emitted beam current of, particularly, frequencies from about tens of Hz to tens of kHz, a medical accelerator system using that accelerator and a method of operating the system.
According to one aspect of the invention to achieve the above object, there is provided a circular type accelerator having deflecting electromagnets and four-pole electromagnets for making a charged particle beam circulate, a multi-pole electromagnet for generating a stability limit of resonance of betatron oscillation in order to produce the charged particle beam, and a high-frequency source for applying a high-frequency electromagnetic field to the charged particle beam to move the charged particle beam to the outside of the stability limit and thereby to excite resonance in the betatron oscillation, characterized in that the high-frequency source generates an AC signal that includes a plurality of different frequency components, the minimum frequency difference of which is in the range from 500 Hz to 10 kHz and, the phases of which include the phase difference between those frequency components and values other than an integerxc3x97xcfx80.
In order to increase the betatron oscillation amplitude of the charged particle beam by high frequencies to shift it to the outside of the stability limit, it is desired that the high frequencies be close to the product of the decimal fraction of the tune (the number of betatron oscillations during the time in which the charged particle beam once circulates in the cyclic type accelerator) of the charged particle beam, and the circulation frequency, or to the product of the decimal fraction of the tune and an integral multiple of the circulation frequency. The tune is changed depending on the amplitude of the betatron oscillation. Thus, in order to exceed the stability limit for irradiation, and hence to increase the amplitude of betatron oscillation, it is necessary to use high frequencies having a plurality of different frequency components.
In the above aspect of the invention, since an AC signal that includes a plurality of different frequency components of which the minimum frequency difference is in the range from 500 Hz to 10 kHz is applied to the charged particle beam from the high-frequency source, the lowest frequency component of the change of the betatron oscillation amplitude of the charged particle beam is in the range from 500 Hz to 10 kHz, and thus it is possible to exclude the change of the irradiation current below some hundreds of Hz that is particularly necessary to be suppressed in the irradiation method in which a small-diameter beam is deflected to scan. In addition, if the phase difference between the frequency components is an integerxc3x97xcfx80, the signal intensity is greatly increased or decreased due to the superimposition of those different frequency components. However, by selecting the phase difference between those frequency components to be a value other than an integerxc3x97xcfx80, it is possible to suppress the emitted beam intensity from changing.
In order to achieve the above object, according to another aspect of the invention, there is provided a cyclic type accelerator having deflecting electromagnets and four-pole electromagnets for making a charged particle beam circulate, a multi-pole electromagnet for generating a stability limit of betatron oscillation resonance for producing the charged particle beam, and a high-frequency source for applying a high-frequency electromagnetic field to the charged particle beam to shift it to the outside of the stability limit and to excite resonance in the betatron oscillation, characterized in that the high-frequency source generates the sum of a plurality of AC signals of which the instantaneous frequencies change with time and of which the average values of the instantaneous frequencies with respect to time are different, and applies the sum signal to the charged particle beam.
When a high-frequency signal having a plurality of frequency components is applied to the charged particle beam, the charged particle beam undergoes the betatron oscillation that has a betatron oscillation frequency (the product of the revolution frequency and tune of the charged particle beam) depending on the intensities of the electromagnets of the accelerator, and the high frequency components applied for emission, and the amplitude of the betatron oscillation is changed at the sum and differences between the betatron oscillation frequency and the high frequency components applied for emission, and at the sums and differences of those high frequency components themselves. As a result, the number of particles of the charged particle beam or the intensity of the emitted charged particle beam, that exceeds the stability limit, is also changed at the same frequencies as above. The frequency components of some tens of kHz or below that are important in the application of the charged particle beam to medical treatment are produced due to the differences between the betatron oscillation frequency and the high frequency components applied for emission, and the differences between those high frequency components for emission. The change of the emitted beam of some tens of kHz or below with time can be reduced on the principle according to the above features of the invention as described below.
The AC signal is expressed by Ai sin(2xcfx80fit+xcex8i) where t is time, Ai the amplitude, and xcex8i the phase, and the instantaneous frequency by fi+(dxcex8i/xc2x7dt)/(2xcfx80). When the instantaneous frequency changes with time, then dxcex8i/dtxe2x89xa00. When the average value of dxcex8i/dt is previously determined to be zero, the average value of the instantaneous frequency with respect to time is fi. The betatron oscillation amplitude of the charged particle beam is changed at the frequency difference between the betatron oscillation frequency and the applied high frequency. According to the above feature, the sum signal, xcexa3Ai sin(2xcfx80fit+xcex8i(t)), of AC signals that have different frequencies fi (i=1, 2 . . . n, where n is 2 or above), and phases xcex8i changing with time, is generated and applied to the charged particle beam.
The betatron oscillation amplitude of the charged particle beam is changed at the difference frequency between the betatron oscillation frequency and the applied high frequency. The betatron oscillation amplitude changes at frequency of fixe2x88x92fxcex2due to the applied high frequency of fi. Since the phase xcex8i of the AC signal of frequency fi changes with time, the phase of the amplitude change of the betatron oscillation at frequency fixe2x88x92fxcex2also depends on the circulation position of the charged particle beam that circulates in the accelerator, that is, on the back-and-forth positions of the beam. As a result, whether the beam is emitted or not depends on the circulation position of the beam that circulates in the accelerator, or on the back-and-forth positions. The direction and position of which the beam that circulates in the accelerator and emitted therefrom are changed at each revolution. In other words, at a certain time, the head of the charged particle beam in the turning direction is emitted, but the second half of the beam from its center in the rotating direction is not emitted. However, as time elapses, the central portion of the beam in the turning direction is emitted, but the first and second halves of the beam in the rotating direction are not emitted. Thus, the betatron oscillation amplitude increases at a different phase depending on the circulation position, and the beam is emitted at a circular position that changes with time. In the prior art, the beam is emitted at all circular positions and similarly less emitted at all circular positions. Therefore, in the invention, the change of all charged particles of the beam with respect to time is extremely small.
According to still another aspect of the invention, there is provided a cyclic type accelerator having deflection electromagnets and four-pole electromagnets for deflecting the charged particle beam to turn, a multi-pole electromagnet for generating a stability limit of resonance of betatron oscillation for the emission of the beam, and a high-frequency source for applying a high-frequency electromagnetic field to the beam to shift it to the outside of the stability limit and hence to excite resonance in betatron oscillation, characterized in that the high-frequency source generates a sum signal of a plurality of different signals whose instantaneous frequencies change with respect to time, and which have average values of the instantaneous frequencies with respect to time, and differences between the instantaneous frequencies and the average values of the instantaneous frequencies with respect to time, and that it applies the sum signal to the beam.
The AC signal is expressed by Ai sin(2xcfx80fit+xcex8i) where t is time, Ai the amplitude, and xcex8i the phase, and the instantaneous frequency by fi+(dxcex8i/dt)/(2xcfx80). When the instantaneous frequency changes with time, dxcex8i/dtxe2x89xa00. When the average value of dxcex8i/dt is previously determined to be zero, the average value of the instantaneous frequency with respect to time is fi. According to the above feature, the sum signal, xcexa3Ai sin(2xcfx80fit+xcex8i(t)), of AC signals of which the (dxcex8i/dt), (dxcex8j/dt) (ixe2x89xa0j) are different, or rates of change of phases xcex8i and xcex8j are different at fi (i=1, 2, . . . n, where n is 2 or above), is generated and applied to the charged particle beam.
The betatron oscillation amplitude of the charged particle beam is changed at the frequency difference between the applied high frequencies. In other words, when the applied frequencies are represented by fi and fj, the betatron oscillation amplitude is changed at the difference fixe2x88x92fj. Also, the phases xcex8i and xcex8j of AC signals of the frequencies fi and fj are changed at different rates with respect to time, and thus the change of the betatron oscillation amplitude at frequency fixe2x88x92fj depends on the circulation position, or phase of the beam that circulates in the accelerator, or on the back-and-forth position of the beam. Thus, since the phase of the increase of the betatron oscillation amplitude depends on the circulation position of the beam, and since the phases change, the number of all charged particles of the beam produced is much less changed with respect to time as in claim 1 of the invention.
According to another aspect of the invention, there is provided a cyclic type accelerator having deflection electromagnets and four-pole electromagnets for making a charged particle beam circulate, a multi-pole electromagnet for generating a stability limit of resonance of betatron oscillation for irradiation of the beam, and a high frequency source for applying a high frequency electromagnetic field to the beam to move the beam to the outside of the stability limit, thus exciting resonance in the betatron oscillation, characterized in that the high frequency source generates a sum signal, xcexa3Ai sin(2xcfx80fit+xcex8i)where t is time, of a plurality of AC signals that have different frequencies fi, and phases xcex8i and amplitude Ai associated with frequencies fi, the phases xcex8i being changed with a predetermined period.
The AC signals are represented by Ai sin(2xcfx80fit+xcex8i) where t is time, and Ai is the amplitude. The instantaneous frequency is expressed by 2xcfx80fi+dxcex8i/dt. Therefore, when xcex8i associated with each fi is changed with a predetermined period as in the characterized-in-that paragraph of the above aspect of the invention, the phase of the increase of the betatron oscillation for irradiation is also changed every second as in the accelerator of claim 1. Thus, the intensity of the produced beam is averaged, with the result that the beam is less changed with respect to time.
According to still another aspect of the invention, there is provided a cyclic type accelerator having deflection electromagnets and four-pole electromagnets for making a charged particle beam circulate, a multi-pole electromagnet for generating a stability limit of resonance of betatron oscillation for irradiation of the beam, and a high frequency source for applying a high frequency electromagnetic field to the beam to move the beam to the outside of the stability limit, thus exciting resonance in the betatron oscillation, characterized in that the high frequency source has a plurality of thermal noise generators, and switching means provided at the stage next to those thermal noise generators in order to select one of the outputs from those generators at predetermined intervals of time, and applies to the beam a high frequency based on the output from the selected thermal noise generator.
Thus, the phase difference between different high frequencies to be applied to the beam is changed with a predetermined period. As a result, the phase of the betatron oscillation amplitude change is changed every second, and hence the produced beam intensity is averaged so that the beam intensity is less changed.
According to another aspect of the invention, there is provided a medical accelerator system having a cyclic type accelerator, a transport system for transporting a charged particle beam produced from the cyclic type accelerator, and an irradiator for irradiating the beam on patient, characterized by the use of the cyclic type accelerator claimed in claim 1 for the accelerator.
Thus, the low frequency components of the amplitude change of the betatron oscillation within the cyclic type accelerator are reduced with the result that the produced beam is less changed with respect to time. Therefore, the beam with its amplitude less changed can be irradiated from the irradiator for treatment.
According to another aspect of the invention, there is provided a medical accelerator system having a cyclic type accelerator, a transport system for transporting a charged particle beam generated from the accelerator, and an irradiator for irradiating the beam on patient, characterized by the use of the cyclic type accelerator claimed in claim 2 for the accelerator.
Thus, the phase of the amplitude change of the betatron oscillation within the cyclic type accelerator is also changed every second, and the generated beam intensity is averaged with the result that the produced beam is less changed with respect to time. Therefore, the beam with its amplitude less changed can be irradiated from the irradiator for treatment.
According to still another aspect of the invention, there is provided a medical accelerator system having a cyclic type accelerator, a transport system for transporting a charged particle beam generated from the accelerator, and an irradiator for irradiating the transported beam on patient, characterized by the use of the cyclic type oscillator claimed in claim 4 for the accelerator.
Thus, the phase of the high frequency to be applied to the beam in order that the beam can be generated from the accelerator is changed with respect to time. Consequently, the phase of the amplitude change of the betatron oscillation is also changed every second, and the produced beam intensity is averaged with the result that the generated beam intensity is less changed with respect to time. Therefore, the beam with its intensity less changed can be irradiated from the irradiator for treatment.
According to another aspect of the invention, there is provided a method of operating a medical accelerator system that has a cyclic type accelerator including deflection electromagnets and four-pole electromagnets for making a charged particle beam circulate, a multi-pole electromagnet for generating a stability limit of resonance of betatron oscillation for irradiation of the charged particle beam, and a high frequency source for applying a high frequency electromagnetic field to the beam to move the beam to the outside of the stability limit, thus exciting resonance in the betatron oscillation; a transport system for transporting the beam produced from the cyclic type accelerator; and an irradiator for irradiating the transported beam on patient, the method comprising the steps of generating from the high frequency source an AC signal for moving the beam to the outside of the stability limit and that includes a plurality of frequency components, between which the minimum frequency difference is in the range from 500 Hz to 10 kHz inclusive, and of which the phases include phase differences between the frequency components and values other than an integerxc3x97xcfx80, applying the AC signal to the beam so that the beam can be generated from the cyclic type accelerator, and irradiating the beam from the irradiator for treatment.
Thus, the low frequency components of the amplitude change of the betatron oscillation within the cyclic type accelerator are reduced, and the produced beam intensity is less changed with respect to time with the result that the beam with its intensity less changed with respect to time can be produced from the accelerator. Therefore, the beam with its amplitude less changed can be irradiated from the irradiator for treatment. Particularly, it is possible to reduce the change of the irradiation current below some hundreds of Hz that is necessary to be suppressed in a small-diameter beam scanning irradiation method.
According to still another aspect of the invention, there is provided a method of operating a medical accelerator system that has a cyclic type accelerator including deflection electromagnets and four-pole electromagnets for making a charged particle beam circulate, a multi-pole electromagnet for generating a stability limit of resonance of betatron oscillation for irradiation of the charged particle beam, and a high frequency source for applying a high frequency electromagnetic field to the beam to move the beam to the outside of the stability limit, thus exciting resonance in the betatron oscillation; a transport system for transporting the beam produced from the cyclic type accelerator; and an irradiator for irradiating the transported beam on patient, the method comprising the steps of generating from the high frequency source a sum signal of a plurality of signals of which the instantaneous frequencies change with respect to time, and of which the average values of the instantaneous frequencies with respect to time are different, applying the sum signal to the beam so that the beam can be produced from the cyclic type accelerator, and irradiating the beam from the irradiator for treatment.
Thus, the phases of a plurality of high frequency components to be applied to the beam in order that the beam can be produced from the accelerator are changed with respect to time. Consequently, the phase of the amplitude change of the betatron oscillation is also changed every second, and the produced beam intensity is averaged so that the beam with its intensity less changed can be generated. Therefore, the beam with its intensity less changed can be irradiated from the irradiator for treatment.
According to further aspect of the invention, there is provided a method of operating a medical accelerator system that has a cyclic type accelerator including deflection electromagnets and four-pole electromagnets for making a charged particle beam circulate, a multi-pole electromagnet for generating a stability limit of resonance of betatron oscillation for irradiation of the charged particle beam, and a high frequency source for applying a high frequency electromagnetic field to the beam to move the beam to the outside of the stability limit, thus exciting resonance in the betatron oscillation; a transport system for transporting the beam produced from the cyclic type accelerator; and an irradiator for irradiating the transported beam on patient, the method comprising the steps of applying to the beam a sum signal, xcexa3Ai sin(2xcfx80fit+xcex8i)where t is time, of a plurality of AC signals that have different high frequencies fi (i=1, 2 . . . n), and phases xcex8i and amplitudes Ai associated with the frequencies fi, the phases xcex8i changing with a predetermined period with respect to time, transporting the beam produced from the accelerator by applying the high frequency signal to the beam, and irradiating the beam from the irradiator.
Thus, the phases of a plurality of high frequencies applied to the beam in order that the beam can be generated from the accelerator are changed at predetermined intervals of time. Consequently, the phase of the amplitude change of the betatron oscillation is changed every second, and the produced beam intensity is averaged with the result that the produced beam intensity is less changed with respect to time. Therefore, the beam with its intensity less changed can be irradiated from the irradiator for treatment.